Patient beds are used anywhere medical care is provided to patients, for example in hospitals and in long-term care facilities, such as palliative care facilities and nursing homes. Patients sometimes fall from their beds, often resulting in injury to the patient. One way of minimizing the likelihood of injury is to reduce the distance between the patient and the floor by providing a height adjustable bed capable of achieving an ultra-low lowermost height position, for example, on the order of 12 to 14 inches between the top of the mattress and the floor. These types of beds will be referred to herein as ultra-low beds. Ultra-low beds typically achieve this ultra-low lowermost position by eliminating the lower secondary frame connecting the legs of the bed that is common in many height adjustable patient beds, especially those used in hospitals. The legs of the bed are instead pivotally attached to the frame of the bed, allowing the legs to collapse beneath the bed when the bed is lowered and permitting the bed to attain the ultra-low lowermost position. An example of such a bed is provided in co-pending commonly held patent application WO 02/26187, filed on Sep. 28, 2001 by the inventor of the present invention, which is hereby incorporated by reference.
Ultra-low beds are typically equipped with caster assemblies at the lower end of each leg to permit the bed to be mobile. The caster assemblies each have a stem for attaching the caster assembly to the bed leg that is offset from the center of the caster wheel. The caster wheel swivels about the stem when the bed is propelled along the floor so that the caster wheel tracks the direction of movement of the bed. This allows the bed to be propelled in any desired direction. In order for the wheel to swivel about the stem, the stem must remain substantially vertical; a non-vertical stem does not allow the caster wheel to track the movement of the bed, thereby restricting the bed to movement only in a single straight line direction. Ultra-low beds are typically mobile only in selected height positions of the bed; for example, the bed disclosed in WO 02/26187, supra, is mobile only in the uppermost height position of the bed and at all other height positions a foot engages the floor to prevent mobility of the bed. However, in some circumstances it is desirable for the bed to be mobile in all height positions, especially when the bed is used in hospitals. Since the legs are pivotally attached to the frame, adjusting the height of the bed causes the lower end of the legs to angularly move with respect to the floor when the height is adjusted. This causes the caster stem to adopt a non-vertical orientation at all but one height position of the bed, preventing the caster wheels from swiveling in order to track the direction of movement of the bed.
The need for maintaining verticality of the caster stem as the height of the bed is adjusted has been identified and dealt with in the prior art. U.S. Pat. No. 6,405,393, filed Dec. 19, 2000 by Megown and issued Jun. 18, 2002 (Megown) discloses a caster support assembly wherein two caster assemblies are provided, one on either side of the pivotal attachment of the leg to the caster support assembly. This causes the caster support assembly to remain in position as the leg angularly moves when the height of the bed is adjusted, thereby maintaining verticality of the caster stem. However, this bed requires two caster assemblies to be attached to the caster support member, which increases the cost of the bed and makes the bed more difficult to maneuver when propelled by an attendant. Also, one of the caster assemblies remains underneath the frame when the bed is lowered to the lowermost position, increasing the minimum distance between the top of the mattress and the floor, which negates some of the advantage of an ultra-low bed in preventing patient injury. European Patent EP 0 558 108, filed Feb. 1, 1993 by applicant Schell Industries BV and granted Sep. 29, 1996 (Schell) discloses a bed with a caster support that is transversely aligned with the frame and attached to a transverse axle that pivots as the height of the bed is adjusted. The caster stem is located on the centerline of the transverse axle so that the caster stem remains vertical as the axle is pivoted. The transverse caster support member offsets the caster assemblies along the width of the bed so that they do not interfere with the frame when the bed is lowered to the lowermost position. This undesirably increases the width of the bed, which can cause problems when negotiating openings such as doorways, etc. with the bed.
The bed disclosed in WO 02/26187, supra, has caster support members which are pivotally attached to the lower end of each leg and longitudinally aligned with the bed frame. The caster assemblies are outward of the frame along the length of the bed, permitting the bed to be lowered to an ultra-low lowermost position without interference between the caster assemblies and the frame and without increasing the overall width of the bed. However, this type of bed suffers from the problem of non-verticality of the caster stem as the height of the bed is adjusted, preventing swiveling of the casters and movement of the bed in any desired direction.
The need therefore exists for an improved ultra-low bed that is steerable in all height positions of the bed.
To further enhance steering of the bed, it is desirable to selectively lock one or more of the caster assemblies at one end of the bed so that the caster assembly or assemblies are prevented from swiveling. The end of the bed with the locked caster assembly or assemblies will then follow the change in direction imparted to the unlocked caster assemblies, allowing the bed to be more easily maneuvered by a single attendant around obstacles and corners. When the bed is pushed by an attendant from one end, it is further desirable for the locked caster assembly to be located at the opposite end of the bed; this allows the bed to be pushed without the opposite end changing direction uncontrollably. Selectively lockable caster assemblies may prevent swiveling of the caster assembly, prevent rotation of the caster wheel, or prevent both swiveling and rotation. Lockable caster assemblies are known in the prior art; see, for example, U.S. Pat. No. 4,998,320, filed Nov. 1, 1989 by Lange and issued Mar. 12, 1991.
Since the caster assembly to be locked is typically located at the end of the bed opposite the attendant, it is desirable for the bed to be equipped with a central actuation mechanism that permits the caster assemblies to be selectively locked from only one end of the bed. Central actuation mechanisms are known on beds having a secondary frame; see, for example, U.S. Pat. No. 6,321,878 filed Mar. 5, 1999 by Mobley, et al. and issued Nov. 27, 2001. However, these types of central actuation mechanisms cannot be readily adapted to ultra-low beds since no secondary frame structure is available for mounting the mechanism and for connecting the front and rear legs. Schell, supra, discloses a central actuation mechanism for preventing rotation of the caster wheels that is electrically powered. In this mechanism, each caster assembly must be electrically actuated, increasing the cost of the bed. Also, the bed requires a source of electrical power to selectively lock the caster assemblies, which may not be available when the bed is being moved.
The need therefore exists for an improved central actuation mechanism for selectively locking one or more caster assemblies on an ultra-low bed.